1. Field of the Invention
The present invention relates to hydrogen-absorbing alloys suitable for use in batteries and in methods of manufacturing batteries. More particularly, the invention relates to Mn-free rare earth-transition metal AB5-type materials which have favorable high rate discharge characteristics and are suitable for use in rechargeable batteries.
2. Description of Related Art
Classes of metal hydride alloys for battery applications include AB alloys (e.g., TiNi), A2B alloys (e.g., Ti2Ni), AB2 alloys (e.g., ZrMn2), AB5 alloys (e.g., LaNi5) and complex alloys (e.g., Vxe2x80x94Tixe2x80x94Zrxe2x80x94Ni). Among these hydrogen-absorbing alloy systems, AB5 is the most attractive alloy system for battery applications because of its favorable environmental characteristics. These alloys lessen the environmental pollution which may be caused by conventional cadmium-based batteries and the like.
LaNi5 has been known to exhibit a theoretical hydrogen storage capacity of about 372 mAh/gm. This high storage capacity enables, in principle, this compound to be an attractive material for the negative electrode of rechargeable batteries. However, the high plateau pressure and poor long-term stability (cycle life) prohibit LaNi5 from practical use in rechargeable batteries. To overcome these drawbacks, many rare earth and transition metal substitutions have been developed to lower the plateau pressure and to extend the cycle life.
Commercial AB5 hydrogen absorbing alloys usually consist of misch metal for the rare earth component (the A part) and Co, Al, Mn and Ni for the transition metal component (the B part). Alloys with such combinations provide increased cycle life at the cost of decreased storage capacity.
Current commercially available AB, hydrogen absorbing alloys are principally designed for consumer electronic appliances, such as telephone devices, which usually require a low discharge current and a low discharge rate. Recently, AB5 alloys have been considered for use in an electrical vehicle (EV) or hybrid electrical vehicles (HEV). For such applications, batteries must be able to release a high amount of current within a very short time period. Thus, a high discharging rate is necessary and required for these applications. To fulfill the high discharging rate requirement for EV or HEV applications, the AB5 alloys must be further modified to allow a fast transport of electrons. It is an object of the present invention to provide AB5 hydrogen absorbing alloys suitable for EV or HEV applications and having a suitably high discharge rate. This and other objects of the present invention will become more apparent from the following description.
The present invention relates to manganese-free rare earth-transition metal AB5-type hydrogen-absorbing alloys having favorable high rate discharge characteristics and being suitable for use in rechargeable batteries, particularly rechargeable batteries for electrical vehicles or hybrid electrical vehicles. Preferably, the hydrogen-absorbing alloys have an AB5 structure, are essentially free of Mn and the MnNi4 phase, and have a unit cell volume ranging from about 87 to about 88.5 A3 at about 25xc2x0 C. More preferably, the hydrogen-absorbing alloys have a plateau pressure of about 1 to about 60 apsi (atmospheric pounds per square inch)(0.075 to 4.5 atm), and more preferably, at 25xc2x0 C., from about 3 to about 10 apsi (0.2 to 0.8 atm).
The hydrogen-absorbing alloy compositions of the present invention are of the general formula: R(CouAlvMwNi1xe2x88x92uxe2x88x92vxe2x88x92w)z, where xe2x80x9cRxe2x80x9d is at least one element selected from the group consisting of rare earth elements and yttrium (Y) (typically La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y), xe2x80x9cMxe2x80x9d is at least one refractory metal selected from Groups IVB, VB, and VIB of the periodic table (typically, M is at least one refractory metal selected from Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W), the value of xe2x80x9cuxe2x80x9d is from 0 to about 0.25, xe2x80x9cvxe2x80x9d is from 0 to about 0.10, xe2x80x9cwxe2x80x9d is from 0 to about 0.05, and the B/A ratio (i.e., (CouAlvMwNi1xe2x88x92uxe2x88x92vxe2x88x92w)/R), xe2x80x9cz,xe2x80x9d is from about 4.70 to about 5.50.